Method for producing silicon wafer

ABSTRACT

The present invention is to produce a silicon crystal wherein the boron concentration in the silicon crystal and the growth condition V/G are controlled so that the boron concentration in the silicon crystal is no less than 1×10 18  atoms/cm 3  and the growth condition V/G falls within the epitaxial defect-free region α 2  whose lower limit line LN 1  is the line indicating that the growth rate V gradually drops as the boron concentration increases. A silicon wafer is also produced wherein the boron concentration in the silicon crystal and the growth condition V/G are controlled so as to include at least the epitaxial defect region β 1 , and both the heat treatment condition and the oxygen concentration of the silicon crystal are controlled so that no OSF nuclei grow to OSFs.

TECHNICAL FIELD

The present invention relates to a method for producing silicon wafers,and more particularly to a method which, during the production of asilicon wafer on which an epitaxial growth layer is formed, can make theepitaxial layer free of defects.

BACKGROUND ART

Silicon crystals are produced by being pulled and grown using theCzochralski (CZ) method. The grown silicon crystal ingot is then slicedinto silicon wafers. Semiconductor devices are fabricated by operationsin which device layers are formed on the surface of the silicon wafer.

However, in the course of silicon crystal growth, crystal defects knownas “grown-in defects” arise.

With the progress in recent years toward higher density and smallergeometries in semiconductor circuits, the presence of such grown-indefects near the surface layer of the silicon wafer where devices arefabricated can no longer be tolerated. This situation has led toexplorations on the possibility of manufacturing defect-free crystals.The following three types of crystal defects are detrimental to devicecharacteristics:

-   a) Crystal originated particles (COP) are void defects which arise    from the coalescing of vacancies;-   b) Oxidation-induced stacking faults (OSF); and-   c) Dislocation loop clusters that arise from the aggregation of    interstitial silicon (also known as interstitial silicon dislocation    defects, or I-defects).

A defect-free silicon monocrystal is recognized or defined as a crystalwhich is free or substantially free of the three above types of defects.

One method for obtaining silicon wafers that are free of grown-indefects near the surface layer where device circuits are created is touse epitaxial growth to grow a defect-free layer on the wafer surface.

That is, an epitaxial silicon wafer is a high value-added silicon wafercarefully created by the vapor-phase growth of an epitaxial growth layer(also known as an “epilayer”) having a high degree of crystal perfectionon a silicon wafer substrate. Because the epitaxial growth layer has ahigh degree of crystal perfection, it is thought to be a substantiallydefect-free layer. Hence, device fabrication on an epitaxial growthlayer dramatically enhances the device characteristics compared withdevice fabrication on the surface layer of a silicon wafer substrate.Because the degree of crystal perfection in the epitaxial growth layerwas not thought to be strongly affected by the crystal qualities of theunderlying silicon wafer substrate, little importance has been placeduntil now on the quality of the silicon wafer substrate itself.

Prior-Art 1

However, in recent years, as the systems used to inspect defects havebecome increasingly sensitive and the criteria for evaluating defectshave become more exacting, it has been found that defects within thesilicon wafer substrate propagate to the epitaxial growth layer, wherethey appear as defects in the epitaxial growth layer (referred to hereinas “epitaxial defects”). This is described in Non-Patent Reference 1(Sato: 16^(th) Meeting of Silicon Technology Division, Japan Society ofApplied Physics; Apr. 24, 2000; p. 35).

Device manufacturers have thus begun calling for the production ofepitaxial defect-free epitaxial silicon wafers having an epitaxialgrowth layer that is free of defects by forming an epitaxial growthlayer on a silicon wafer substrate which is free of crystal defects thatcause epitaxial defects.

Grown-in defects in a silicon wafer substrate include defects whichreadily propagate to the epitaxial growth layer and defects which do notreadily propagate. OSFs and dislocation loop clusters in particular arevery likely to propagate to the epitaxial growth layer and becomeepitaxial defects, and so must be excluded from the silicon wafersubstrate.

If the temperature gradient G in the crystal axis (vertical) directionis assumed to be constant, the defects in a silicon monocrystal varywith the pull rate V of the silicon monocrystal. In other words, it isknown that, as the pull rate V decreases from a high speed, void defects(COPs), OSFs (ring-like OSFs, abbreviated as “Ring-OSF,” which arestacking faults observed on a ring concentric with the center of thewafer following heat treatment in an oxidizing atmosphere), defect-freeregions and dislocation loop clusters arise one after another.

In P-type silicon crystals, boron (B) is added to the silicon crystal asa dopant. In p/p⁺ and p/p⁺⁺ epitaxial silicon wafers containing a highconcentration of boron, about 1×10¹⁸ atoms/cm³ to 1×10¹⁹ atoms/cm³ ofboron have been added to the silicon crystal.

Prior-Art 2

Non-Patent Reference 2 (E. Dornberger, E. Graff, D. Suhren, M. Lambert,U. Wagner and W. von Ammon: Journal of Crystal Growth, 180 (1997), 343)describes the influence of boron on the behavior of crystal defects.This Non-Patent Reference 2 discloses that adding boron to a highconcentration in a silicon crystal results in the generation of R-OSFsat a higher pull rate V.

Conditions currently used for the production of p⁺ and p⁺⁺ siliconcrystals are described here with referring to attached diagramsaccording to the present invention.

FIG. 2A shows the distribution of epitaxial defect regions and epitaxialdefect-free regions. The vertical axis represents the normalized pullrate V/Vcri when the temperature gradient G in the crystal axis(vertical) direction is assumed to be constant, and the horizontal axisrepresents the concentration of boron added to the silicon crystal inatoms/cm³. The normalized pull rate V/Vcri refers herein to the pullrate normalized by the critical rate Vcri when the added boronconcentration is 1×10¹⁷ atoms/cm³, and the critical rate Vcri refers tothe pull rate at which R-OSFs at the center of the silicon crystaldisappear when the pull rate V is gradually decreased.

Epitaxial defect-free region α1 in FIG. 2A is an epitaxial defect-freeregion where void defects appear in the silicon wafer substrate and theepitaxial growth layer is free of defects. Epitaxial defect region β1 isan epitaxial defect region where OSFs appear in the silicon wafersubstrate and defects appear in the epitaxial growth layer. Epitaxialdefect-free region α2 is an epitaxial defect-free region where thesilicon wafer substrate is free of defects and the epitaxial growthlayer is free of defects. Epitaxial defect region β2 is an epitaxialdefect region where dislocation loop clusters appear in the siliconwafer substrate and defects appear in the epitaxial growth layer.

Up until now, p⁺ silicon crystals have been produced in the regionindicated as J (called the production conditions region) in FIG. 2A. Theproduction conditions region J includes epitaxial defect region β1. Inorder to suppress epitaxial defects, attempts have been made to producesilicon crystals within epitaxial defect-free region α2 by moving theproduction conditions region to a lower V side—that is, to theproduction conditions region K shown in FIG. 2B.

Prior-Art 3

Here, in low boron concentration p⁻ silicon crystals (having a boronconcentration of less than 1×10¹⁸ atoms/cm³), when the pull rate V islowered, defects arise in the epitaxial growth layer due to dislocationloop clusters. However, in high boron concentration p⁺ and p⁺⁺ siliconcrystals, Non-Patent Reference 3 (Asayama, et al.: 1999 Fall Meeting ofJapan Society of Applied Physics; 3p-ZY-4) reports the suppression ofdislocation loop clusters even at the same low pull rate V.

Accordingly, it was previously thought that, in the production ofhigh-boron concentration p⁺ and p⁺⁺ silicon crystals, lowering the pullrate V would enable the relatively easy production of high-qualitysilicon crystals without epitaxial defects. That is, it was predictedthat a lower limit for the epitaxial defect-free region α2 exists at lowboron concentrations (less than 1×10¹⁸ atoms/cm³), but does not exist athigh boron concentrations (1×10¹⁸ atoms/cm³ to 1×10¹⁹ atoms/cm³).

DISCLOSURE OF THE INVENTION

Contrary to the above prediction, we have found that when the pull rateV is lowered, dislocation loop clusters which can cause epitaxialdefects arise even in high boron concentration p⁺ and p⁺⁺ siliconcrystals.

With the foregoing in view, a first object of the present invention isto determine the lower limit LN1 of epitaxial defect-free region α2 andthereby make it possible to manufacture in a good yield high-qualityepitaxial silicon wafers which are free of epitaxial defects.

With regard to epitaxial defect region β1, because OSFs which occur inthe silicon wafer substrate propagate to the epitaxial growth layerwhere they appear as defects, the conventional thinking up until nowheld that silicon crystals should be produced in a production conditionsregion which does not include this region.

However, we have discovered that, even when a silicon crystal has beenproduced in a production conditions region that includes epitaxialdefect region β1, depending on the process conditions, no defects appearin the epitaxial growth layer.

In light of these circumstances, a second object of the invention is toenable high-quality epitaxial silicon wafers which are free of epitaxialdefects to be produced in a good yield within a production conditionsregion that includes epitaxial defect region β1.

Also, epitaxial defect-free region α1 has been thought of as a regionwhere void defects arise in the silicon wafer substrate, but defects donot appear in the epitaxial growth layer.

However, device manufacturers lately have been calling for the formationof epitaxial growth layers as thin films having a thickness of no morethan 2 μm. We have found that defects caused by void defects which werethought not to appear in epitaxial growth layers having an ordinary filmthickness (about 5 μm) do appear as epitaxial defects in the epitaxialgrowth layer when it is formed as such a thin film.

In light of the foregoing, a third object of the invention is to make itpossible to produce in a good yield high-quality, epitaxial defect-freeepitaxial silicon wafers within epitaxial defect-free region α1, evenwhen the epitaxial growth layer is formed as a thin film.

A first aspect of the invention is characterized by a method forproducing a silicon wafer which comprises:

a silicon crystal production step of producing a silicon crystal whilecontrolling a concentration of boron in the silicon crystal and a growthcondition V/G (where V is a growth rate, and G is a temperature gradientin a crystal axis direction) so as to fall within an epitaxialdefect-free region (α2) which is a defect-free region where a siliconwafer substrate is free of defects and an epitaxial growth layer is freeof defects and which has a lower limit line (LN1) where, at the boronconcentration in the silicon crystal of 1×10¹⁸ atoms/cm³ and above, thegrowth rate V gradually decreases as the boron concentration rises;

a silicon wafer substrate obtaining step of obtaining the silicon wafersubstrate from the silicon crystal; and

an epitaxial growth step of forming the epitaxial growth layer on thesilicon wafer substrate.

A second aspect of the invention is characterized in, in the siliconcrystal production step of the silicon wafer production method accordingto the first aspect of the invention, making the temperature gradient Gin the silicon crystal axis direction uniform to within a given degreebetween a center of the crystal and an edge of the crystal.

A third aspect of the invention is characterized in, in the siliconcrystal production step of the silicon wafer production method accordingto the second aspect of the invention, applying a magnetic field to thesilicon melt from which the silicon crystal is pulled, thereby makingthe temperature gradient G in the silicon crystal axis direction uniformto within a given degree between the center of the crystal and the edgeof the crystal.

A fourth aspect of the invention is characterized in, in the siliconcrystal production step of the silicon wafer production method accordingto the second aspect of the invention, bringing the silicon melt fromwhich the silicon crystal is pulled to a magnetic field-free state andcontrolling the number of rotations of the silicon crystal, therebymaking the temperature gradient G in the silicon crystal axis directionuniform to within a given degree between the center of the crystal andthe edge of the crystal.

A fifth aspect of the invention is characterized in, in the siliconcrystal production step of the silicon wafer production method accordingto the second aspect of the invention, bringing the silicon melt fromwhich the silicon crystal is pulled to a magnetic field-free state andcontrolling the number of rotations of a quartz crucible holding thesilicon melt, thereby making the temperature gradient G in the siliconcrystal axis direction uniform to within a given degree between thecenter of the crystal and the edge of the crystal.

A sixth aspect of the invention is characterized in, in the siliconcrystal production step of the silicon wafer production method accordingto the first aspect of the invention, controlling the oxygenconcentration in the silicon crystal to no more than 12.5×10¹⁷atoms/cm³.

A seventh aspect of the invention is characterized in, in the siliconcrystal production step of the silicon wafer production method accordingto the second aspect of the invention, controlling the oxygenconcentration in the silicon crystal to no more than 12.5×10¹⁷atoms/cm³.

According to the first aspect of the invention, a silicon crystal isproduced by, as indicated by LN1 in FIG. 1, controlling theconcentration of boron in the silicon crystal and the growth conditionV/G (where V is the growth rate, and G is the temperature gradient inthe crystal axis direction) so as to fall within an epitaxialdefect-free region (α2) which is a defect-free region where a siliconwafer substrate is free of defects and an epitaxial growth layer is freeof defects and which has a lower limit line (LN1) where, at boronconcentrations in the silicon crystal of 1×10¹⁸ atoms/cm³ and above, thegrowth rate V gradually decreases as the boron concentration rises.

Next, a silicon wafer substrate is obtained from the silicon crystalthat has been produced.

Next, an epitaxial growth layer is formed on the silicon wafersubstrate.

In this way, there is produced a high-quality epitaxial silicon waferwherein defects appear neither in the silicon wafer substrate nor in theepitaxial growth layer.

Because this first aspect of the invention clearly indicates the lowerlimit LN1 of the epitaxial defect-free region α2, boron concentrationsand growth conditions V/G which do not fall below the lower limit LN1can be precisely determined. By producing a silicon crystal at a boronconcentration and a growth condition V/G that do not fall below thelower limit LN1, high-quality epitaxial silicon wafers which are free ofepitaxial defects can be manufactured in a high yield.

The second aspect of the invention adds a technical limitation to thesilicon crystal production step in the first aspect of the invention.Namely, in the silicon crystal production step, the temperature gradientG in the silicon crystal axis direction is made uniform to within agiven degree between a center of the crystal and an edge of the crystal.

As shown in FIG. 3, the uniformity of the temperature gradient G in thesilicon crystal axis direction determines the vertical extent of theproduction conditions region (the extent in the direction of thevertical axis V/Vcri) for the silicon crystal. The more uniform thetemperature gradient G in the silicon crystal axis direction between thecenter 10 c of the silicon crystal and the edge 10 e of the crystal, thesmaller the vertical extents Be to Bc and Ac to Ae of the productionconditions region in FIG. 3 can be made, allowing the productionconditions region to more easily fit within epitaxial defect-free regionα2 and enabling the production of high-quality epitaxial silicon wafersfree of epitaxial defects in a higher yield.

The third aspect of the invention adds another technical limitation tothe silicon crystal production step in the second aspect of theinvention. Namely, in the silicon crystal production step, a magneticfield is applied to the silicon melt from which the silicon crystal ispulled so as to make the temperature gradient G in the silicon crystalaxis direction uniform to within a given degree between the center ofthe crystal and the edge of the crystal.

The fourth aspect of the invention adds yet another technical limitationto the silicon crystal production step in the second aspect of theinvention. In the silicon crystal production step, the silicon melt fromwhich the silicon crystal is pulled is placed in a non-magnetic fieldstate and the rate of rotation of the silicon crystal is controlled soas to make the temperature gradient G in the silicon crystal axisdirection uniform to within a given degree between the center of thecrystal and the edge of the crystal.

The fifth aspect of the invention adds a yet further technicallimitation to the silicon crystal production step in the second aspectof the invention. Namely, in the silicon crystal production step, thesilicon melt from which the silicon crystal is pulled is placed in anon-magentic field state and the number of rotations of a quartzcrucible holding the silicon melt is controlled so as to make thetemperature gradient G in the silicon crystal axis direction uniform towithin a given degree between the center of the crystal and the edge ofthe crystal.

The sixth and seventh aspects of the invention add a still furthertechnical limitation to the silicon crystal production step in,respectively, the first aspect and second aspect of the invention.Namely, in the silicon crystal production step, the oxygen concentrationin the silicon crystal is controlled to no more than 12.5×10¹⁷atoms/cm³.

By controlling the oxygen concentration in the silicon crystal to a lowlevel of no more than 12.5×10¹⁷ atoms/cm³ according to the sixth andseventh aspects of the invention, even if the production conditionsregion extends into epitaxial defect region β1, no OSF nuclei in thesilicon wafer substrate readily grow into OSFs and appear in theepitaxial growth layer as epitaxial defects. As a result, the conditionsfor selecting the boron concentration and the growth condition V/G canbe relaxed, enabling the yield to be improved even further.

An eighth aspect of the invention is characterized in a method forproducing a silicon wafer, which comprises a step of controlling theboron concentration in the silicon crystal and the growth condition V/G(where V is the growth rate, and G is the temperature gradient in thecrystal axis direction) so as to include at least an epitaxial defectregion (β1) where oxidation-induced stacking faults (OSF) occur in asilicon wafer substrate and defects occur in an epitaxial growth layer,and in controlling the silicon crystal heat treatment conditions and theoxygen concentration in the silicon crystal so that no OSF nucleidevelop into OSFs.

According to the eighth aspect of the invention, even if the productionconditions region is a range which includes the epitaxial defect regionβ1, by controlling the silicon crystal heat treatment conditions and theoxygen concentration in the silicon crystals, no OSF nuclei in thesilicon wafer substrate grow into OSFs and no epitaxial defects appearin the epitaxial growth layer. Hence, high-quality epitaxial siliconwafers can be produced in a high yield in a production conditions regionhaving a high pull rate V.

A ninth aspect of the invention is characterized in a method forproducing a silicon wafer, which comprises:

a silicon crystal production step of producing a silicon crystal whilecontrolling the boron concentration in the silicon crystal and thegrowth condition V/G (where V is the growth rate, and G is thetemperature gradient in the crystal axis direction) so as to fall in thevicinity of a lower limit line (LN3) within an epitaxial defect-freeregion (α1) where void defects occur in a silicon wafer substrate and anepitaxial growth layer is free of defects;

a silicon wafer substrate obtaining step of obtaining a silicon wafersubstrate from the silicon crystal; and

an epitaxial growth step of forming a thin-film epitaxial growth layerof no more than 2 μm on the silicon wafer substrate.

A tenth aspect of the invention is characterized in, in the siliconcrystal production step of the silicon wafer production method accordingto the ninth aspect of the invention, controlling the oxygenconcentration in the silicon crystal to no more than 12.5×10¹⁷atoms/cm³.

The ninth aspect of the invention was arrived at based on the discoverythat void defects (COPs) are smaller in size and number near the lowerlimit line LN3 within epitaxial defect-free region al than in regionsaway from the lower limit line LN3. By controlling the boronconcentration and growth condition V/G within the silicon crystal andthereby setting the production conditions region near the lower limitline LN3 within epitaxial defect-free region α1, the size and number ofvoid defects (COPs) become smaller. Therefore, even when the epitaxialgrowth layer is formed as a thin film having a thickness of 2 μm orless, void defects within the silicon wafer substrate do not propagateto the epitaxial growth layer and clearly exist as epitaxial defects. Asa result, high-quality, thin-film epitaxial silicon wafers can beproduced in a good yield within a production conditions region having ahigh pull rate V.

By controlling the oxygen concentration within the silicon crystal to alow concentration of 12.5×10¹⁷ atoms/cm³ or less in accordance with thetenth aspect of the invention, even if the crystal production conditionsregion extends into the epitaxial defect region β1, no OSF nuclei in thesilicon wafer substrate grow into OSFs and appear as epitaxial defectsin the epitaxial growth layer. The conditions for setting the boronconcentration and growth condition V/G can thus be relaxed, enabling theproduction yield to be increased even further.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing the relationship, according to oneembodiment of the invention, between the boron concentration (atoms/cm³)in a silicon crystal 10, the growth condition V/Vcri (growth conditionV/G), epitaxial defect regions, and epitaxial defect-free regions;

FIGS. 2A and 2B are diagrams which correspond to FIG. 1 and show therelationship with target production conditions regions;

FIG. 3A is a diagram which corresponds to FIG. 1 and shows therelationship with the vertical extents Be to Bc and Ac to Ae of theproduction conditions regions, FIG. 3B is a diagram which shows thedistribution in the crystal radial direction of the temperature gradientG in the crystal axis direction near the solid-liquid interface, FIG. 3Cis a diagram showing various isotherms in the crystal near thesolid-liquid interface;

FIG. 4 shows the construction of a silicon crystal production apparatus(single crystal silicon growth system) such as may be used in thesilicon wafer production method of the invention;

FIG. 5A illustrates the convexity of the solid-liquid interface, FIG. 5Bis a diagram showing the experimental results obtained from aninvestigation of the change in convexity of the solid-liquid interfaceversus the crystal pulling conditions; and

FIG. 6 is a diagram showing the experimental results obtained from aninvestigation of the change in convexity of the solid-liquid interfaceversus the number of rotations of the crucible.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the silicon wafer production methods of the invention aredescribed below in conjunction with the attached diagrams.

FIG. 4 is a side view showing the construction of a silicon crystalproduction apparatus such as may be used in the embodiments of theinvention.

Referring to FIG. 4, a single crystal puller 1 according to thisembodiment has a CZ furnace (chamber) 2 as the single crystal pullingenclosure.

Inside the CZ furnace 2, there is provided a quartz crucible 3 whichmelts the polysilicon starting material and holds it as a melt 5. Thequartz crucible 3 is covered on the outside with a graphite crucible 11.A main heater 9 which heats and melts the polysilicon starting materialinside the quartz crucible 3 is provided outside of and lateral to thequartz crucible 3. Below the quartz crucible 3, there is provided anauxiliary heater (bottom heater) 19 which additionally heats the base ofthe quartz crucible 3 to prevent solidification of the melt 5 at thebase of the quartz crucible 3. The outputs (power; kW) of the mainheater 9 and auxiliary heater 19 are independently controlled, therebyindependently adjusting the heating rate of the melt 5. For example, thetemperature of the melt 5 is detected and the detected temperature isused as a feedback value to control the respective outputs of the mainheater 9 and the auxiliary heater 19 so that the melt 5 reaches a targettemperature.

A heat-insulating tube 13 is provided between the main heater 9 and theinside wall of the CZ furnace 2.

A pulling mechanism 4 is provided above the quartz crucible 3. Thepulling mechanism 4 includes a pull shaft 4 a and a seed chuck 4 c atthe end of the pull shaft 4 a. The seed chuck 4 c holds a seed crystal14.

Polysilicon (Si) is heated and melted within the quartz crucible 3. Oncethe temperature of the melt 5 stabilizes, the pulling mechanism 4operates, pulling monocrystal silicon 10 (referred to hereinafter as“silicon crystal 10”) from the melt 5. That is, the pull shaft 4 adescends, immersing the seed crystal 14 held by the seed chuck 4 c atthe end of the pull shaft 4 a into the melt 5. After the seed crystal 14has been allowed to adjust to the melt 5, the pull shaft 4 a is raised.As the seed crystal 14 held by the seed chuck 4 c rises, the siliconcrystal 10 grows. During pulling, the quartz crucible 3 is rotated at aspin rate ω1 by a rotating shaft 110. The pull shaft 4 a of the pullingmechanism 4 is rotated at a spin rate ω2 in either the oppositedirection or the same direction as the rotating shaft 110.

In this embodiment, boron (B) is added beforehand to the melt 5 in thequartz crucible 3 so as to incorporate boron in the silicon crystal 10to be pulled.

The rotating shaft 110 can be driven in the vertical direction so as tomove the quartz crucible 3 up or down to a desired position. By shuttingthe interior of the CZ furnace 2 off from outside air, a vacuum (e.g.,about 20 Torr) within the furnace 2 is maintained. That is, argon gas 7is supplied as an inert gas to the CZ furnace 2, then the CZ furnace 2is evacuated through an exhaust port by a pump. The pressure within thefurnace 2 is lowered in this way to a predetermined value.

In the course of the single crystal pulling process (1 batch), variousevaporants arise within the CZ furnace 2. To remove these evaporants andthereby clean the CZ furnace 2, argon gas 7 is fed to the CZ furnace 2then discharged outside of the CZ furnace 2 together with theevaporants. The feed rate of the argon gas 7 is set separately for eachstep in a single batch.

As the silicon crystal 10 is pulled, the amount of melt 5 decreases. Asthe amount of melt 5 decreases, the contact area between the melt 5 andthe quartz crucible 3 changes, resulting in a change in the amount ofoxygen dissolution from the quartz crucible 3. This change affects thedistribution of the oxygen concentration within the pulled siliconcrystal 10. To prevent this, additional polysilicon starting material ormonocrystal silicon starting material is added, either during or afterpulling, to the quartz crucible 3 in which the amount of melt 5 hasdecreased.

Above the quartz crucible 3, a heat shield 8 (gas flow regulating tube)shaped approximately as the inverted frustum of a cone is providedaround the silicon crystal 10. The heat shield 8 is supported by theheat-insulating tube 13. This heat shield 8 directs argon gas 7 as thecarrier gas supplied into the CZ furnace 2 from above to the center ofthe melt surface 5 a, causing the gas to pass over the melt surface 5 aand reach the periphery thereof The argon gas 7 is then discharged,together with evaporated gases from the melt 5, through an exhaust portprovided at the bottom of the CZ furnace 2. The gas flow rate at thesurface of the liquid can thus be stabilized, enabling oxygen thatevaporates from the melt 5 to be kept in a stable state.

The heat shield 8 also insulates and shields the seed crystal 14 and thesilicon crystal 10 that grows from the seed crystal 14 from the radiantheat that emerges in high-temperature areas such as the quartz crucible3, melt 5 and main heater 9. Furthermore, the heat shield 8 preventsimpurities (e.g., silicon oxides) generated within the furnace fromdepositing on the silicon crystal 10 and impeding development of thesingle crystal. The size of the gap H between the bottom edge of theheat shield 8 and the melt surface 5 a can be adjusted by raising andlowering the rotating shaft 110 and thereby changing the verticalposition of the quartz crucible 3. The gap H can also be adjusted bymoving the heat shield 8 upward or downward using a raising and loweringmechanism.

The growth condition V/G (where V is the growth rate, and G is thetemperature gradient in the crystal axis direction) of the siliconcrystal 10 is controlled by adjusting the gap H and the pull rate V bythe pull shaft 4 a.

The boron concentration (amount of boron added, atoms/cm³) within thesilicon crystal 10 is controlled by adjusting the amount of boronintroduced to the quartz crucible 3.

The oxygen concentration (atoms/cm³) within the silicon crystal 10 iscontrolled by regulating such parameters as the crucible spin rate ω1,the pulling shaft spin rate ω2, the argon gas flow rate and the pressurewithin the furnace.

The ingot of silicon crystal 10 produced by the apparatus in FIG. 4 isthen sliced with a slicing machine to give a silicon wafer.

The silicon wafer is placed within the furnace of an epitaxial growthsystem, and a gas serving as the starting material for the thin layer,such as trichlorosilane (SiHCl₃) is supplied to the surface of thesilicon wafer. Chemical reaction of the tridhlorosilane then causes athin film of the same silicon to form by epitaxial growth on the surfaceof the silicon wafer substrate. A crystal having the same arrangement ofatoms as the silicon wafer substrate is formed in this way as anepitaxial film on the substrate.

FIG. 1 shows the relationship, according to the present embodiment,between the boron concentration (atoms/cm³) in the silicon crystal 10,the growth condition V/G, and the epitaxial defect regions and epitaxialdefect-free regions.

The vertical axis in FIG. 1 represents the normalized pull rate V/Vcriwhen the temperature gradient G in the crystal vertical (axis) directionis assumed to be constant, and the horizontal axis represents theconcentration of boron (atoms/cm³) added to the silicon crystal 10.Here, the normalized pull rate VNcri is the pull rate normalized by thecritical rate Vcri when the concentration of boron added is 1×10¹⁷atoms/cm³, and the critical rate Vcri is the pull rate at the time thatR-OSFs disappear at the center of the silicon crystal when the pull rateV is gradually decreased.

Epitaxial defect-free region α1 in FIG. 1 is an epitaxial defect-freeregion where void defects appear in the silicon wafer substrate and theepitaxial growth layer is free of defects. Epitaxial defect region β1 isan epitaxial defect region where OSFs appear in the silicon wafersubstrate and defects appear in the epitaxial growth layer. Epitaxialdefect-free region α2 is an epitaxial defect-free region where thesilicon wafer substrate is free of defects and the epitaxial growthlayer is free of defects. Epitaxial defect region β2 is an epitaxialdefect region where dislocation loop clusters appear in the siliconwafer substrate and defects appear in the epitaxial growth layer.

FIG. 1 is explained by way of comparison with the prior art.

According to Prior-Art 3, it was predicted that epitaxial,defect-freeregion β2 has a lower limit at low boron concentrations (less than1×10¹⁸ atoms/cm³), but does not have a lower limit at high boronconcentrations (1×10¹⁸ atoms/cm³ to 1×10¹⁹ atoms/cm³).

However, contrary to the above prediction, we have found that when thepull rate V is lowered, dislocation loop clusters which can causeepitaxial defects arise even in high boron concentration p⁺ and p⁺⁺silicon crystals.

That is, we discovered the line indicated by LN1 in FIG. 1 where, atboron concentrations in the silicon crystal of 1×10¹⁸ atoms/cm³ andabove, the growth rate V gradually falls as the boron concentrationincreases to be the lower limit line of epitaxial defect-free region α2.

Production Method 1

Silicon crystal 10 is thus produced while controlling the boronconcentration within the silicon crystal 10 and the growth condition V/G(where V is the growth rate, and G is the temperature gradient in thecrystal axis direction) so as to lie within epitaxial defect-free regionα2 and not fall below the lower limit line LN1.

Next, a silicon wafer substrate is obtained from the silicon crystal 10thus produced.

Next, an epitaxial growth layer is formed on the silicon wafer substratethus obtained.

As a result, there is produced a high-quality epitaxial silicon wafer inwhich defects appear in neither the silicon wafer substrate nor theepitaxial growth layer.

Defect characterization can be carried out, for example, by x-raytopography following copper (Cu) decoration. Various other techniquesmay also be used, such as infrared scattering, x-ray topographyfollowing oxygen precipitation heat treatment, etching opticalmicroscopy following oxygen precipitation heat treatment, and Seccoetch. Combinations of these techniques may also be used.

According to the present embodiment, the lower limit LN1 of epitaxialdefect-free region α2 is clearly delineated, enabling boronconcentrations and growth conditions V/G which do not fall below thelower limit LN1 to be precisely determined. By producing a siliconcrystal 10 at a boron concentration and a growth condition V/G which donot fall below the lower limit LN1, high-quality epitaxial siliconwafers that are free of epitaxial defects can be produced in a goodyield.

Embodiments such as the following in which controls have been added toforegoing Production Method 1 are also possible.

Production Method 2

In this Production Method 2, a control is added to make the temperaturegradient G in the silicon crystal 10 axis direction when a siliconcrystal 10 is produced by Production Method 1 uniform to within a givendegree from the center of the crystal 10 c to the crystal edge 10 e.

FIG. 2B shows an example of a production conditions region K when asilicon crystal 10 is produced by Production Method 1.

We have found that the vertical extent of production conditions region Kin FIG. 2B is determined by the uniformity in the crystal radialdirection of the temperature gradient G in the silicon crystal 10 axisdirection, and the temperature gradient G in the crystal axis directionbecomes uniform by giving the solid-liquid interface which is theboundary between the melt 5 and the silicon crystal 10 during pulling ofthe silicon crystal an upwardly convex shape.

FIG. 3A is a diagram which corresponds to FIG. 1. In FIG. 3A, thevertical extents of the production conditions regions are designatedrespectively as Be to Bc and Ac to Ae.

FIG. 3C shows the isotherms L1, L2, L3, L4 and L5 near the solid-liquidinterface which is the boundary between the melt 5 and the siliconcrystal 10 during pulling of the silicon crystal. In FIG. 3C, theisotherm L1 is an isotherm at the solid-liquid interface, and L2, L3, L4and L5 are isotherms at positions of successively far from thesolid-liquid interface in the crystal axis direction. Isotherms L1, L2,L3, L4 and L5 represent isotherms between the center 10 c and the edge10 e (periphery) of the silicon crystal 10. The distance between thecenter 10 c and edge 10 e corresponds to the radius R of the siliconcrystal 10. In FIG. 3C, the solid-liquid interface has an upwardlyconvex shape, along with which the temperature gradient G in the axialdirection of the silicon crystal 10 can be seen to be uniform at eachposition r from the center 10 c to the edge 10 e of the crystal 10.

FIG. 3B shows the distribution in the crystal radius direction of thetemperature gradient G in the direction of the crystal axis near thesolid-liquid interface. The horizontal axis in FIG. 3B represents theradial position r/R in the radial direction of the silicon crystal 10normalized by the radius R of the silicon crystal 10, and the verticalaxis represents the temperature gradient G in the crystal axis directionnear the solid-liquid interface.

Line A in FIG. 3B represents a case in which the temperature gradient Aein the crystal axis direction at the edge 10 e of the crystal is largerthan the temperature gradient Ac in the crystal axis direction at thecenter 10 c of the crystal. Line B in FIG. 3B represents a case in whichthe temperature gradient Be in the crystal axis direction at the edge 10e of the crystal is smaller than the temperature gradient Bc in thecrystal axis direction at the center 10 c of the crystal. Moreover, inthis case, all of line A has a larger temperature gradient G in thecrystal axis direction than line B.

As can be seen on comparing FIG. 3A with FIG. 3B, in FIG. 3A, theproduction conditions region Be to Bc throughout which the temperaturegradient G in the crystal axis direction is smaller is situated abovethe production conditions region Ac to Ae throughout which thetemperature gradient G in the crystal axis direction is larger. That is,as the temperature gradient G in the crystal axis direction becomessmaller, production conditions region Ac to Ae or Be to Bc approachesthe upper limit LN2 of epitaxial defect-free region α2 (lower limit ofepitaxial defect region β1), and as the temperature gradient G in thecrystal axis direction becomes larger, the production conditions regionAc to Ae or Be to Bc approaches the lower limit LN1 of epitaxialdefect-free region α2 (upper limit of epitaxial defect region β2).

Also, by having the temperature gradient Ae in the crystal axisdirection at the edge 10 e of the crystal be larger than the temperaturegradient Ac in the crystal axis direction at the center 10 c of thecrystal, the crystal center 10 c (Ac) having a small temperaturegradient G in the crystal axis direction can be positioned above in FIG.3A and the crystal edge 10 e (Ae) having a larger temperature gradient Gin the crystal axis direction can be positioned below in FIG. 3A.Similarly, by having the temperature gradient Be in the crystal axisdirection at the edge 10 e of the crystal be smaller than thetemperature gradient Bc in the crystal axis direction at the center 10 cof the crystal, the crystal edge 10 e (Be) having a smaller temperaturegradient G in the crystal axis direction can be positioned above in FIG.3A and the crystal center 10 c (Bc) having a larger temperature gradientG in the crystal axis direction can be position below in FIG. 3A.

The flatter line A in FIG. 3B becomes, i.e., the more uniform thetemperature gradient G in the crystal axis direction at each positionr/R from the center Ac to the edge Ae of the crystal, the smaller thevertical extent Ac to Ae of the production conditions region in FIG. 3Acan be made. Likewise, the flatter line B in FIG. 3B becomes, i.e., themore uniform the temperature gradient G in the crystal axis direction ateach position r/R from the center Bc to the edge Be of the crystal, thesmaller the vertical extent Bc to Be of the production conditions regionin FIG. 3A can be made.

Here, as shown in FIG. 3C, the temperature gradient G in the crystalaxis direction becomes uniform by giving the solid-liquid interface anupwardly convex shape.

Therefore, in this embodiment, control is carried out so as to give thesolid-liquid interface an upwardly convex shape, thereby making thetemperature gradient G in the crystal axis direction uniform. As aresult, the vertical extent Ac to Ae of the production conditions regionor the vertical extent Be to Bc of the production conditions region inFIG. 3A becomes smaller. As the vertical extent Ac to Ae of theproduction conditions region in FIG. 3A becomes smaller, the edge 10 e(Ae) of the crystal situated on the lower side in the diagram can beprevented from falling below the lower limit LN1 of epitaxialdefect-free region α2 and entering epitaxial defect region β2.Similarly, as the vertical extent Be to Bc of the production conditionsregion in FIG. 3A becomes smaller, the edge 10 e (Be) of the crystalsituated on the upper side in the diagram can be prevented fromexceeding the upper limit LN2 of epitaxial defect-free region α2 andentering epitaxial defect region β1.

Thus, in the present embodiment, the target production conditions regionK in FIG. 2B can easily be made to fit within epitaxial defect-freeregion α2, enabling epitaxial defect-free, high-quality epitaxialsilicon wafers to be produced in a higher yield.

Production Method 3

Next, an example of control to give the solid-liquid interface anupwardly convex shape is described.

In this Production Method 3, during the production of a silicon crystal10, a magnetic field is applied to the silicon melt 5 from which thesilicon crystal 10 is pulled, thereby giving the solid-liquid interfacean upwardly convex shape and making the temperature gradient G in theaxial direction of the silicon crystal 10 uniform to within a givendegree between the crystal center 10 c and the crystal edge 10 e.

Exemplary methods for applying a magnetic field to the melt 5 includeusing a superconducting magnet to apply a horizontal magnetic field orto apply a cusp magnetic field.

Applying a magnetic field to the melt 5 suppresses convection within themelt 5. As a result, the heating rate by the main heater 9 forcontrolling the solid-liquid interface to the target temperature (e.g.,1412° C.) increases. In turn, the amount of heat which enters thesolid-liquid interface from the melt 5 increases, giving thesolid-liquid interface an upwardly convex shape.

Production Method 4

Next, another example of control to give the solid-liquid interface anupwardly convex shape is described.

In this Production Method 4, during the production of a silicon crystal10, the silicon melt 5 from which the silicon crystal 10 is pulled isplaced in a magnetic field-free state and the spin rate ω2 by thesilicon crystal is controlled so as to make the temperature gradient Gin the axis direction of the silicon crystal 10 uniform to within agiven degree between the crystal center 10 c and the crystal edge 10 e.

Raising the spin rate ω2 of the silicon crystal 10 above a fixed levelcauses an upwardly swirling current to arise within the melt 5,activating heat transport at the center of the melt 5. This gives thesolid-liquid interface an upwardly convex shape.

FIG. 5B shows the experimental results obtained from a study of thechange in the convex shape of the solid-liquid interface versus thecrystal pulling conditions when a 200 mm diameter silicon crystal 10 waspulled. The horizontal axis in FIG. 5B represents the pull rate V, andthe vertical axis shows the center height (amount of protrusion) Xcen ofthe solid-liquid interface. When the center height (amount ofprotrusion) Xcen of the solid-liquid interface is a positive value, thesolid-liquid interface is upwardly convex; when the center height(amount of protrusion) Xcen of the solid-liquid interface is a negativevalue, the solid-liquid interface is downwardly convex. The centerheight (amount of protrusion) Xcen of the solid-liquid interface isdefined in FIG. 5A.

In FIG. 5B, S/R 26 represents a case in which the spin rate ω2 of thesilicon crystal 10 was 26 rpm, S/R 30 represents a case in which thespin rate ω2 of the silicon crystal 10 was 30 rpm, H30 represents a casein which the gap H between the bottom edge of the heat shield 8 and themelt surface 5 a was 30 mm, and H50 represents a case in which this gapH was 50 mm. A magnetic field was not applied.

Production Method 5

To give the solid-liquid interface an upwardly convex shape, it is alsopossible to control the spin rate ω1 of the quartz crucible 3 instead ofcontrolling the spin rate ω2 of the silicon crystal 10.

In this Production Method 5, during the production of a silicon crystal10, by placing the silicon melt 5 from which the silicon crystal 10 ispulled in a magnetic field-free state and controlling the spin rate ω1of the quartz crucible 3, the temperature gradient G in the siliconcrystal 10 axis direction is made uniform to within a given degreebetween the center 10 c of the crystal and the edge 10 e of the crystal.

FIG. 6 shows the experimental results obtained from a study of thechange in the convex shape of the solid-liquid interface versus the spinrate ω1 of the quartz crucible 3. The horizontal axis in FIG. 6represents the spin rate ω1 of the quartz crucible 3, and the verticalaxis represents the center height (amount of protrusion) Xcen of thesolid-liquid interface. The pull rate was 1.5 mm/min. A magnetic fieldwas not applied.

Production Method 6

In this Production Example 6, during production of a silicon crystal 10in Production Method 1, control of the oxygen concentration in thesilicon crystal 10 to no more than 12.5×10¹⁷ atoms/cm³ is added.

In Production Method 1, it is assumed that the production conditionsregion K falls within epitaxial defect-free region α2, as shown in FIG.2B. However, in some cases, the production conditions are relaxed andthe silicon crystal 10 is produced in a production conditions regionwhich extends into epitaxial defect region β1.

Here, control is carried out limiting the oxygen concentration in thesilicon crystal 10 to no more than 12.5×10¹⁷ atoms/cm³. When the siliconcrystal 10 has a low oxygen concentration, even if the crystalproduction conditions region extends into epitaxial defect region β1,OSF nuclei in the silicon wafer substrate do not readily grow into OSFdefects and appear as epitaxial defects in the epitaxial growth layer.This allows the conditions for setting the boron concentration and thegrowth condition V/G to be relaxed, enabling the production yield to befurther enhanced.

Production Method 7

In this Production Method 7, as shown in FIG. 2A, a silicon crystal 10is produced by controlling the boron concentration within the siliconcrystal 10 and the growth condition V/G such as to give a productionconditions region J which at least includes epitaxial defect region β1.Here, “at least includes epitaxial defect region β1” refers to cases inwhich the production conditions region fits inside epitaxial defectregion β1, cases in which it extends into both epitaxial defect regionβ1 and epitaxial defect-free region α1, cases in which it extends intoboth epitaxial defect region β1 and epitaxial defect-free region α2, andcases in which it extends into epitaxial defect region β1, epitaxialdefect-free region α1 and epitaxial defect-free region α2.

Moreover, in Production method 7, the oxygen concentration in thesilicon crystal 10 is controlled and the silicon wafer substrate is heattreated so as to keep OSF nuclei from developing into OSFs.

Oxygen concentration and heat treatment conditions to keep OSF nucleifrom developing into OSFs are listed below.

-   1) Controlling the oxygen concentration in the silicon crystal 10 to    no more than 12.5×10¹⁷ atoms/cm³, and administering heat treatment    in a dry O₂ gas atmosphere at 1000° C. for 16 hours.-   2) Controlling the oxygen concentration in the silicon crystal 10 to    no more than 11 atoms/cm³, and administering heat treatment in a wet    O₂ gas atmosphere at 650° C. for 3 hours and at 1100° C. for 2    hours.-   3) Controlling the oxygen concentration in the silicon crystal 10 to    no more than 11 atoms/cm³, and administering heat treatment in a dry    O₂ gas atmosphere at 650° C. for 3 hours and at 1000° C. for 16    hours.

When the oxygen concentration in the silicon crystal 10 was controlledand the silicon wafer substrate was heat treated under oxygenconcentration and heat treatment conditions such as those indicatedabove, the OSF nuclei in the silicon wafer substrate did not grow intoOSFs and appear as defects in the epitaxial growth layer.

In this embodiment, epitaxial defects originating from OSFs do not ariseeven when the production conditions region is a range that includesepitaxial defect region β1. Hence, high-quality epitaxial silicon waferscan be produced in a good yield within a production conditions region Jhaving a high pull rate V like that shown in FIG. 2A.

Production Method 8

We have discovered that, on approaching the lower limit line LN3 withinepitaxial defect-free region α1, the size and number of void defects(COP) diminish. We learned that when thin-film epitaxial silicon wafersare produced, void defects propagate to the epitaxial growth layer anddevelop into epitaxial defects in the upper left-hand region of thediagram away from the lower limit line LN3 of the epitaxial defect-freeregion α1, but epitaxial defects originating from void defects do notdevelop in the region near the lower limit line LN3 inside epitaxialdefect-free region α1.

Hence, this Production Method 8 produces a silicon crystal 10 bycontrolling the boron concentration in the silicon crystal 10 and thegrowth condition V/G so as to give a production conditions region nearlower limit line LN3 yet within epitaxial defect-free region α1.

A silicon wafer substrate is then obtained from the silicon crystal 10thus produced.

A thin-film epitaxial growth layer having a thickness of 2 μm or less isthen formed on the silicon wafer substrate thus obtained.

Epitaxial defect-free, high-quality thin-film epitaxial silicon wafersare thus produced. According to this embodiment, high-quality thin-filmepitaxial silicon wafers can be produced in a high yield at a high pullrate V.

Production Method 9

In this Production Method 9, during production of the silicon crystal 10by Production Method 8, a control is added so as to limit the oxygenconcentration in the silicon crystal 10 to no more than 12.5×10¹⁷atoms/cm³.

In this Production Method 9, during production of the silicon crystal 10by Production Method 8, a control is added so as to limit the oxygenconcentration in the silicon crystal 10 to no more than 12.5×10¹⁷atoms/cm³.

Hence, control is carried out which limits the oxygen concentration inthe silicon crystal 10 to no more than 12.5×10¹⁷ atoms/cm³. By givingthe silicon crystal 10 a low oxygen concentration, even if the crystalproduction conditions region should extend into epitaxial defect regionβ1, no OSF nuclei in the silicon wafer substrate grow into OSF defectsand appear in the epitaxial growth layer as epitaxial defects. The boronconcentration and the growth condition V/G setting conditions can thusbe relaxed, enabling further improvement in the production yield.

1. A method for producing a silicon wafer, comprising: a silicon crystalproduction step of producing a silicon crystal while controlling aconcentration of boron in the silicon crystal and a growth condition V/G(where V is a growth rate, and G is a temperature gradient in a crystalaxis direction) by using, as boundary condition, a prescribedrelationship between the boron concentration in the silicon crystal andthe growth condition V/G (where V is the growth rate, and G is thetemperature gradient in the crystal axis direction), which is shown by alower limit line (LN1) of an epitaxial defect-free region (α2), so as tofall within the epitaxial defect-free region (α2) which is a defect-freeregion in which a silicon wafer substrate is free of defects and anepitaxial growth layer is free of defects and which has the lower limitline (LN1) in which, at the boron concentrations in the silicon crystalof 1×10¹⁸ atoms/cm³ and above, the growth rate V gradually decreases asthe boron concentration rises, and so as not to fall within an epitaxialdefect region (β2) in which a dislocation loop cluster occurs in thesilicon wafer substrate and defects occur in the epitaxial growth layerand in which the lower limit line (LN1) is an upper limit line; asilicon wafer substrate obtaining step of obtaining the silicon wafersubstrate from the silicon crystal; and an epitaxial growth step offorming the epitaxial growth layer on the silicon wafer substrate,wherein in the silicon crystal production step, controlling the oxygenconcentration in the silicon crystal to no more than 12.5×10¹⁷atoms/cm³.
 2. The silicon wafer production method according to claim 1,characterized in, in the silicon crystal production step, controlling tomake the temperature gradient G in the silicon crystal axis directionuniform between a center of the crystal and an edge of the crystal tosuch an extent that a region between the center of the crystal and theedge of the crystal does fall under the lower limit line (LN1) of theepitaxial defect-free region (α2).
 3. The silicon wafer productionmethod according to claim 2, characterized in, in the silicon crystalproduction step, applying a magnetic field to a silicon melt from whichthe silicon crystal is pulled, thereby controlling to make thetemperature gradient G in the silicon crystal axis direction uniformbetween the center of the crystal and the edge of the crystal.
 4. Thesilicon wafer production method according to claim 2, characterized in,in the silicon crystal production step, bringing the silicon melt fromwhich the silicon crystal is pulled to a magnetic field-free state andcontrolling the number of rotations of the silicon crystal, therebycontrolling to make the temperature gradient G in the silicon crystalaxis direction uniform between the center of the crystal and the edge ofthe crystal.
 5. The silicon wafer production method according to claim2, characterized in, in the silicon crystal production step, bringingthe silicon melt from which the silicon crystal is pulled to a magneticfield-free state and controlling the number of rotations of a quartzcrucible holding the silicon melt, thereby controlling to make thetemperature gradient G in the silicon crystal axis direction uniformbetween the center of the crystal and the edge of the crystal.
 6. Amethod for producing a silicon wafer, comprising: controlling a boronconcentration in a silicon crystal and a growth condition V/G (where Vis a growth rate, and G is a temperature gradient in a crystal axisdirection) so as to include at least an epitaxial defect region (β1) inwhich oxidation-induced stacking faults (OSF) occur in a silicon wafersubstrate and defects occur in an epitaxial growth layer; performingheat treatment on the silicon crystal; and controlling the oxygenconcentration in the silicon crystal no more than 12.5×10¹⁷ atoms/cm³ sothat no OSF nuclei develop into OSFs.
 7. A method for producing asilicon wafer, comprising: a silicon crystal production step ofproducing a silicon crystal while controlling a boron concentration inthe silicon crystal and a growth condition V/G (where V is a growthrate, and G is a temperature gradient in a crystal axis direction) so asto fall in an epitaxial defect-free region (α1) in which void defectsoccur in a silicon wafer substrate and epitaxial defects that are acause of the void defects, do not appear on an epitaxial growth layerafter forming a thin film epitaxial growth layer of less than 2 μm onthe silicon wafer substrate where the void defects occur; a siliconwafer substrate obtaining step of obtaining the silicon wafer substratefrom the silicon crystal; and an epitaxial growth step of forming thethin-film epitaxial growth layer of no more than 2 μm on the siliconwafer substrate, wherein in the silicon crystal production step,controlling the oxygen concentration in the silicon crystal to no morethan 12.5×10¹⁷ atoms/cm³.